The effect of glycine receptor activation on neurite outgrowth and survival was studied in 5 DIV (days in vitro) spinal neurons. These neurons were depolarized by spontaneous synaptic activity and by glycine, but not by glutamate. These responses were accompanied by increases in intracellular calcium concentration measured with Indo-1 and Fluo-3. Glycine (100 microM, 48 h) increased (46 +/- 6%) the number of primary neurites and total neuritic length. This effect was mediated by synaptic activity and calcium influx because TTX (1 microM) and nimodipine (4 microM) blocked the stimulatory effect of glycine. Neuronal survival, on the other hand, was not affected. This study shows for the first time the modulatory effect of glycine receptors on spinal neuron development.